Here's a long question for the experienced drivers amongst us, wasn't sure whether to put this in the advanced driving sub-forum

I've previously noticed that my car feels like the front end is very light under heavy braking. From my limited experience, it feels like the steering wheel has been disconnected, similar to losing grip in snow or trying to go round a roundabout whilst driving on a diesel spill.

Until now I've put this down to being typical behaviour of my car (Mazda3) and ABS, something I've not had before. I've not had to perform a real emergency braking manouvere and I only use heavy braking for fun.

So the problem, when braking very, very heavily at the weekend, dry road, straight line, slightly downhill, nothing else about, the car weaved left then right before stopping. It was maybe only a couple of feet one way and then back but it was a large enough movement to notice. I assume it was using the ABS at the time but I didn't notice as I was concentrating on the alarming movement. I was holding the wheel firmly and there was no pull on the wheel at the time.

I would assume that the wheels lost grip as in the snow/diesel situation but why? If the front wheels do lose grip, I can't see the point of having ABS as the car is not under control.

I would have expected a modern FWD car with disc brakes front and back, ABS and some fancy brakeforce distribution system to be perfectly stable under straight line ABS braking so that I could steer the thing slightly but it seems this is not the case. Any thoughts?

Wrt the lurching, something similar was demonstrated to me on a handling day. Apparently the length of pipe to each brake caliper is not necessarily the same so when there's a sudden application of the brake, the fluid can cause some wheels to brake earlier than other. This is quite alarming

The way round it is to "hint" at the brakes by slightly depressing the pedal, this loads up all the brakes, before pushing the pedal hard, all the brakes are applied evenly and the car will stop straight and true (assuming that tracking, etc is correct), and in a shorter distance than a normal emergency stop. For comparisons of ways to bring a car to a halt, I'd recommend Andy Walsh's Bending the Rules dvd

Chris

Much better described in this online book: Don Palmer's Driving Handbook

Chris

The car should pull up straight. Furthermore, the ABS would 'normally' virtually guarantee this in almost any conditions.

Suspect (as you do) something is amiss and need sorting PDQ.

I would have cr*pped myself if my car had darted 2 feet to one side and then back - defeats the whole object of ABS.

The movement was a gentle drift rather than a pull or a lurch.

This being a bit of road I quite often do this, I usually start braking gently and then fully depress the pedal, rather than just stamping on them - if that were to matter on the brake fluid effect. Initially I thought the light feeling had something to do with the slope. But in a braking situation, shouldn't all the weight be on the front wheels?

Also, I have been able to replicate the light feeling on a level road, although I've not managed to properly test the weaving part.

This sounds like a geometric and weight-transfer effect, rather than anything to do with the anti-lock. Get hold of a pre-downforce film of sports-racers (or even F1 cars) under heavy braking, say Le Mans up to about 1972 (including, or even especially the Steve McQueen film), and watch the cars weave. Now get a recent on-board of a racing sidecar outfit, and watch how much more stable it is with the passenger transferring weight to the back of the outfit.

I'd agree with Chris about "finding the balance point of the brakes" though (which is how Jackie Stewart explained it to me).

Are you sure you didn't have a tiny bit of lock on at the time?

The front should go "heavy" under braking due to the weight transfer, which suggests that something is not as it should be.

The "darting" could be down to road irregularities ie lumps, bumps, camber etc. At higher speeds these will all be more noticeable. Tyres can also have an impact, are they worn? Are they equal pressures across the axle?

From what you are saying I'm not sure your ABS is working. Do you, under hard braking in wet and greasy conditions (a classic ABS activator is a large wet grid) get a horrible 'grating' / 'kicking back' feel from the pedal ?

Does the Mazda have a ABS light - i.e. one that flickers on when the ABS is in use ?

Thanks for the replies. I don't think there is something wrong with the car as it has been fine until now and is perfectly predictable in normal driving. Although I would like to establish what is happening in this extreme situation "I don't like the way it acts in an emergency stop from 70+" is up there with "the dashboard squeaks at 90" in warranty claims I won't be making unless I can show that there's something not working right.

In no particular order:

ABS - no flashing light when it activates, no obvious feedback through the pedal but there a horrible grating/grinding noise. I have only once managed to get some tyre squeal so I've assumed that the ABS works. Is a lack of skid marks the only way to tell your ABS is working?

Tyres are in good conditon, even pressure, plenty of tread etc. I've checked them

No steering input at the time, as I was driving straight before braking and holding the wheel firmly in preparation for stopping.

Tracking seemingly ok as it drives true on the motorway, wheels should be balanced, no rim mounted weights to fall/kerb off

Road conditions were good and no obvious surface contamination or damage. I previously experienced the lightness but not the weaving in the same spot.

One thing that has occurred to me is that despite being only 100hp and similar lb/ft of torque, the car will noticably torque steer when provoked, first time it happened I really didn't expect it. In the situation described, I was in 3rd gear (usually in 4th) so higher up the rev band. What happens when you brake while the engine is almost at full tilt? Torque steer on braking? If sudden acceleration can do it, likewise sudden deceleration? Without giving me any steering wheel feedback? Increased momentum means grip is lost easily rather than when accelerating and tyre grip digs in and pulls the car left?

I think this is natural.

Just because it has ABS doesnt mean it will always pull up straight. As sadi before the road conditions at time would have played a part. Mybe the road was sweeping off to left (as most roads do slightly). This would affect weight distribution, and therefore breaking. Also, a bit of loose rubble on road (not normally noticeable) may have hampered the ABS a little.

Remember ABS is just locking and releasing of brakes, if something is making one tyre have more grip than the other the breaking will be affected and the car wont stop straight without correction.

At a recent track day present in a Mitsubishi Evo and Scooby STi both wobbled and weaved a little under very heavy breaking (ABS all the way ) and needed constant minor adjustments.

However, if the effect is against the camber of the road, or is ( as it seems) causing undue nervousness! in you you should get it checked.

Take a few test drives and see if other cars feel the same...

HTH

>Torque steer on braking? If sudden acceleration can do it, likewise sudden deceleration?

Probably, had a Panda hire car many moons ago and this exhibited this behaviour (but more drastically) - frightened the wits out of me.

Surprised you're not getting the slight kickback/ vibration through the brake pedal but the graunching grating noise sounds right. Sounds as if the ABS is working - is there nothing in the manual about a light flickering on to alert you, would have thought this was standard practice these days ?

Next question is - if you're not at full tilt in 3rd - i.e. mebbe on light throttle or overun and you brake hard, do you still get the same symptoms ?

If not then it would seem to be the torque steer effect after all.

Surprised you're not getting the slight kickback/ vibration through the brake pedal but the graunching grating noise sounds right.

Sorry, I'm getting confused myself now, you can feel the pulsing through the brake pedal

At a recent track day present in a Mitsubishi Evo and Scooby STi both wobbled and weaved a little under very heavy breaking (ABS all the way ) and needed constant minor adjustments.

However, if the effect is against the camber of the road, or is ( as it seems) causing undue nervousness! in you you should get it checked.

I'm not unduly concerned that my car is broken/unsafe, just surprised that it did this for the first time from not completely stupid speeds. I shall go away and play around with it. Weathers turned wet so I'll be waiting till it drys up.

I would go for road camber/ imperfections/ suface changes/ ect ect to cause the effect you describe......

AFAIK the light does not flash when the ABS is working , TCS lamp will flash when operating though

on the Mazda 3s i've road tested, PDI'd , serviced etc , i've never noticed any undue problems when braking and can't remember a customer ever complaining about a problem , but if you are unsure about the ABS operation get it checked out

Sorry, I'm getting confused myself now, you can feel the pulsing through the brake pedal

On my last car you could feel a slight but very rapid vibration - after all the brakes are being applied on and off very rapidly.